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Full text of "The Cambridge Natural History"

26                                        LOWER   JAW

ossification, may be added). The angular, splenial, and all the
other elements of the reptilian jaw have vanished, though the
numerous points from which the mammalian dentary ossifies
is a reminiscence of a former state of affairs ; and here again
an occasional continuance of the separation is preserved, as the
case observed by Professor Albrecht of a separate supra-angular
"bone in a Rorqual attests. Among other reptilian bones that are
not to be found in the mammalian skull are the basipterygoids,
quadrato-jugal, and supratemporal. A few of these bones,
however, though no longer traceable in the adult skull save in
cases of what we term abnormalities, do find their representatives
in the foetal skull Professor Parker, for example, has described
a supra-orbital in the embryo Hedgehog; a supratemporal also
appears to be occasionally independent.
In the mode of the articulation of the lower jaw to the skull
the Mammalia apparently, perhaps really, differ from other
Vertebrates. In the Amphibia and Beptilia, with which groups
alone any comparisons are profitable, the lower jaw articulates
by means of a quadrate bone, which may be movably or firmly
attached to the skull. In the mammals the articulation of the
lower jaw is with the squainosaL The nature of this articulation
is one of the most debated points in comparative anatomy.
Seeing that Professor Kangsley1 in the most recent contribution
to the subject quotes no less than fifty-two different views, many
of which are more or less convergent, it will be obvious that in a
work like the present the matter cannot be treated exhaustively.
As, however, Professor Eangsley justly says that ** no single bone
occupies a more important position in the discussion of the
origin of the Mammalia than does the quadrate," and with equal
justice adds that " upon the answer given as to its fate in this
group depends, in large measure, the broader problem of the
phylogeny of the Mammalia/' it becomes, or indeed has long been,
a matter which cannot be ignored in any work dealing with the
mammals. A simple view, due to the late Dr. Baur and to
Professor Dollo, commends itself at first sight as meeting the
case. The last-named author holds, or held, that in all the
higher Vertebrates it is at least on a priori grounds likely
that two such characteristically vertebrate features as the
lower jaw and the chain of bones bringing the outer world
1  Tufts College Studies, 'No. &> 1900,